It is widely believed that fossil fuels will be consumed soon. On the other hand, the greenhouse gases such as carbon dioxides and methane generated from fossil fuel plants would lead global warming. Therefore, there are world-wide efforts to develop environmentally friendly energy and devices with low energy consumption and/or driven by renewable energy such as solar energy. Photovoltaic (PV) cell is one of the important devices to replace fossil fuel in electricity generation. To date, commercial available photovoltaic cells are made from inorganic semiconductors such as Si, CdTe, CuInxGa1-xSe. Particularly, poly-silicon (poly-Si) and single crystal silicon (SC-Si) together contributed to nearly 90% of the market share. However, the high processing cost of inorganic semiconductors has posted a drawback in the development of solar cell industry.
In contrast, as organic photovoltaic (OPV) cells are of potential low cost, ease of process in large-scale production and compatibility on flexible substrates of organic semiconductors, many attractions have been drawn. OPV cell is an optoelectronic device comprises at least one component that utilize organic or organometallic small molecules or polymeric materials for light absorption and charge process. Harnessing the power of chemical synthesis, a large variety of organic molecules or polymers with different band gaps and absorption coefficients can be synthesized to maximize the light absorption and power generated from the photovoltaic cells. Thus, the organic photovoltaic cell has emerged as a new class of solar cell technologies.
Within the area of organic photovoltaic cells, various device architectures have been explored including the dye-sensitized solar cell (DSSC), organic/inorganic hybrid organic cells, and organic photovoltaic cells with heterojunctions. In 1986, C. W. Tang found that bilayer heterojunction structure fabricated from copper phthalocyanine (CuPc) and perylene tetracarboxylic derivative gives a power conversion efficiency (PCE) of 0.95%. Afterward, small-molecule donor materials such as pentacene, tetracene, metal phthalocyanines (Pcs) were widely studied.
Later, as polymeric materials show good device performance, most affords have been shifted to develop polymeric donor materials. Recently, the PCE of OPV with polymeric donor materials is approaching the practical value of 10%. Compare with the silicon-based solar cells, as the lower efficiency and shorter lifetime of the OPV can be compensated by their low cost at this value, OPV is ready for commercialization. (Prof. Photovolt. Res, Appl. 2012, 29, 377). In literature, PTB7 is one of the best polymeric donor materials. Using PTB7, a base value of 6.22% PCE has been obtained with a simple device fabrication method and architecture. (Adv. Mater. 2010, 22, E135) Using advanced device fabrication method, pro-treatment and different device architecture, the PCE of PTB7-OPV has been largely enhanced. (Adv. Mater. 2010, 22, E135; Nature Photonics 2012, 6, 591) The best value of 9.2% has been achieved by Prof. Cao Yong using an inverted structure. (Nature Photonics 2012, 6, 591)
On the other hand, it is widely accepted that, polymeric donors such as PTB7 suffer from 1) do not have well-defined molecular structures; 2) difficult to obtain high purity without batch-to-batch variation; 3) difficult to obtain material with high carrier mobility and 4) contain end groups as contaminants. These points have been summarized by Prof. Cao. (Adv. Mater. 2013, DOI:10:1002/adma.201301716) Therefore, the focus moved back to small-molecule donor materials. In 2011, Prof. Tang fabricated an OPV with small-molecule donor which gave a power conversion efficiency of 5.23%. (Adv. Mater. 2011, 23, 4960). In 2012, Prof. Nguyen developed a new small-molecule donor (p-DTS(FBTTh2)2). The device fabricated show a PCE of 1.8% without any post-treatment, and the best value of 5.8% was achieved after a 130° C. post-deposition annealing. (Adv. Mater. 2012, 24, 3646).